With the ever increasing volume of traffic on roadways, there is a need for more efficient and safer traffic management. The need for local, unidirectional or bidirectional communication, involving specific vehicles and specific information, or between the roadside and vehicles, has been accomplished by various schemes. Some schemes include directional antennas, selected radio frequencies, both continuous and pulsed, and signal encoding. Electronic license plates have also been developed for such local communication. Electronic license plates are license plates that provide both visual information as traditional license plates provide, as well as electronic information and communication. The electronic information can be used for automatic vehicle identification for toll collection purposes or for identification of special vehicles, such as buses to improve scheduling, and police cars and ambulances to improve the efficiency of dispatching such vehicles. Highway vehicles can be electronically monitored by law enforcement agencies. Currently, police use license plates to visually identify vehicles. With electronic license plates, police can use the visual information in conjunction with radio frequency interrogation systems to electronically request and receive the same information provided visually by the license plate, as well as other information, such as the validity of required on-board documents, such as insurance, registration or emission certificates. Other uses of systems using the electronic communication capabilities of electronic license plates include automatic restriction of areas to certain vehicles, traffic control, vehicle theft protection, toll collection, collision avoidance and emergency message communication.
One way to provide electronic information in addition to the visual information is to provide an electronic module in a vehicle for electronically communicating with an interrogator. For example, in U.K. Patent Application GB 2,256,072A to Matsushita Electric Industrial Co. Ltd., a road antenna from an automatic vehicle identification system receives, by radio, data from an ID plate which is separate from the license plate and is located inside the vehicle. While having an electronic module separate from the license plate accomplishes the goal of providing both electronic and visual information, it is further desirable to have both integrated into a single unit for ease of installation, the ability to use existing structures on vehicles for installation, to optimize antenna location for low power communication between the roadside and the vehicle and to ensure that the visual and electronic information is consistent.
An example of an integrated electronic license plate wherein both visual and electronic information are provided in a unitary structure is disclosed in U.S. Pat. No. 4,001,822 to Sterzer. The Sterzer electronic license plate has a plurality of layers. The top layer is an electrically non-conductive layer having visual information printed on it where the printing is similar to the printing on a conventional license plate. Behind the top layer are two antenna networks disposed on both sides of a dielectric substrate and another non-conducting layer behind the dielectric substrate. The antenna networks can receive an interrogation signal from a microwave transmitter and transmit an identification signal in response to the interrogation signal. The Sterzer license plate derives a harmonic signal from the interrogation signal and re-radiates the identification signal at the harmonic frequency.
The Sterzer electronic license plate uses a printed circuit antenna array having a plurality of dipoles with a corporate feed interconnecting the dipole elements. Microstrip antennas are metallic patches printed on a thin, grounded dielectric substrate. To transmit an electromagnetic signal, a feed network feeds a RF signal generated by connected external electronics, to the metallic patch which resonates and sets up an electromagnetic field between the patch and the ground plane and radiates electromagnetic energy. Conversely, a microstrip antenna receives a signal when electromagnetic energy radiated from a source strikes the metal patches, induces and electric field at its resonant frequency and transforms it to an electrical signal.
Visual communication of specific motor vehicle identification has long been accomplished by a license plate with a signature of the state and year of registration along with a unique identification number. In the past, the surface of the license plate was painted in a bright color for ease of reading the visual information. The painted surface was a metal plate to allow stamping of the alphanumerics and to provide mechanical rigidity. The optical efficiency of locating and reading license plates, however, has been significantly enhanced by use of an optically retroreflective surface such as 3M brand Scotchlite.TM. Reflective License Plate Sheeting, manufactured by Minnesota Mining and Manufacturing Company of St. Paul, Minn., and printed with the appropriate vehicle designation. The enclosed lens retroreflective surface is a composite material having a monolayer of microspheres, such as glass beads, embedded in a matrix of resin material for positioning, physical integrity and protection and having a reflective coating underlying and spaced from the glass beads. The space coat is made up of an array of connected hemispheres with the substantially continuous metal reflective coating, preferably vapor deposited aluminum, underlying the space coat. The glass bead and reflective surface coating construction provides for reflection of incident light, such as an automobile headlight, to be reflected back at the source, thereby making the surface very visible and readable. The retroreflective construction is then laminated to a metal, usually aluminum, substrate for mechanical stability and for mounting the license plate.
It is well known that the transmission of radio signals is distorted, if not completely inhibited, when an interposing conductive plane is placed in the path between a transmitter and receiver. An interposing conductive plane closely located to a microstrip antenna renders the antenna ineffective for the frequency it was designed for because it detunes the operational frequency of the antenna thereby preventing the antenna from resonating at the antenna's operational or resonant frequency. Also, the radiating field radiates into the interposing conductive layer rather than out into the preferred direction. The conductive layer then converts the energy into an electrical signal and reradiates the energy, resulting in a distorted field pattern and loss of the desired directionality and gain.
In prior art integrated electronic license plates, the visual layer of the license plate had a painted visual surface to aid optical reading of the vehicle's information. The retroreflective sheeting used with traditional license plates could not be used because its continuous vapor coat layer acted as an interposing conductive plane when placed on the surface of the electronic license plate, such as if placed on the first planar member of the Sterzer electronic license plate. The continuous vapor coat layer would have distorted or blocked the fields radiated from the antenna network and rendered the antenna network behind the retroreflective sheeting ineffective for communication. Therefore, what is desirable is an integrated electronic license plate capable of electromagnetic communication and having a retroreflective surface for improved optical visibility.
Similar to electronic license plates, electronic road signs have been developed for unidirectional or bidirectional communication between vehicles and the roadside, such as for in-vehicle signing. An electronic road sign has a surface with printed visual information for visual communication of desired information, such as toll collection, traffic control information or dangerous condition warnings. The electronic road sign also has a transmitter to electronically transmit information associated with the road sign. In-vehicle signing allows a vehicle on the roadway to electronically receive the information from the roadside transmitter and display the information inside the vehicle. Also, the information can be communicated by audio within the vehicle. The electronic road sign may also have a receiver to receive information from the vehicle, such as in toll collection to verify transactions, or from traffic management centers to update the message information to be delivered to a vehicle.
Electronic road signs are desirable because they effectively communicate information to vehicles in situations where visibility between the vehicle and the road sign is poor. The electronic road sign can provide redundant information to ensure the driver of a vehicle receives the information. For example, German Patent Application DE 41 42 091 A1 to Siemens Matsushita Components, filed Aug. 5, 1993, describes an identification system for recognizing traffic information in vehicles, where an active transponder is placed inside a vehicle and a passive transponder is provided near a traffic sign, such as under the sign and mounted on the same post. What is further desirable is a retroreflective electronic road sign that enhances the optical visibility of the road sign, further improving the communication of information. Even more desirable is a integrated retroreflective electronic road sign. While a separate antenna may be placed alongside a road sign, or on the same mounting as a road sign, it is preferable to integrate the antenna or receiver with the road sign to prevent sign congestion on the road side, to be able to use existing infrastructure to implement the electronic road signs, for ease of installation, to reduce cost and for safety considerations.